Top

# Chromophore

When light applied to target tissue, the chromophore helps in absorbing these light energy within cellular organs especially mitochondria. A chromophore is part of the molecular structure responsible for colour. So anything that changes colour when exposed to light or responds to light in a different manner than its original form definitely shows the presence of chromophore in the tissue. The chromophore polymer composite are chromophores physically incorporated into commercially available polymer materials such as amorphous polycarbonate etc.

Haemoglobin lycopene and beta carotene are good examples of chromophores and the physiological effects of these chromophore activation are mostly related to cellular level conditions. According to IUPAC chromophores are group of atoms or groups within a molecule which provides specific colors to the external features of the molecule.

## Chromophore Definition

Apart from the IUPAC definition where the chromophore has been described as specialised cells or atoms which helps in pertaining certain colour to the molecule, this also covers two important aspects.

One is related to the system’s response to an external perturbation or the spectrum, through which the chromophore is linked to the experimental behaviour of molecular system. Secondly, it also helps in understanding the localised excitement states which helps controlling the tentative response to the spectrum.

The modern day theory of McWeeny helps in getting a better picture of electronic structure of the chromophore where electron groups are involved.
If the basic description of the electronic absorption process is to be understood, the electromagnetic radiation or spectrum of wavelengths in the range of 180 nm to approximately 800 nm, interact through rapid varying electric field with the electronic charge distribution that defines shape, size and energy of the chromophore.

The magnetic field of light can also interact with a chromophore which is typically more than 106 times weaker than the interaction with the electric field and has a critical role to play in circular dichroism spectroscopy.

The influence can be seen directly in absorption spectra when the electric interaction is weak. The parity of the electric and magnetic dipole operators are found to be odd and even respectively which helps them segregating properly.

The chromophore can also be defined as covalently unsaturated group which are responsible for electronic absorption. The colours usually appears in an organic compound if it contains certain unsaturated groups. These unsaturated groups are called chromophores and any compound having these chromophore are called chromogen.

These are basically saturated groups with the non-bonded electrons which when gets bonded to a chromophore leads to alteration of both the wavelength as well as the intensity of the absorption. But they themselves cannot impart colour to any compound.

The auxochromes are either acidic or basic and usually are salt forming groups such as $–NH_{2}$, -OH or even soluble radicals like –COOH or $-SO_{3}H$. These auxochromes generally deepen the colour of a chromogen but they themselves have any colour of their own.

Any functional group which is responsible for imparting colour to the compound was originally called as chromophore, like the nitro compounds are yellow due to presence of $–NO_{2}$ group as chromophore.

## Example of Chromophore Group

The concept of a chromophore is analogous to that of group vibration and just as the wavenumber of a group vibration is treated as transferable from one molecule to another, so is the wave number at which the electronic transition occurs in a particular group. Such a group is called a chromophore since it results in a characteristics colour of the compound due to absorption of visible or widening the use of ultra violet radiation.

Ethylene group: whatever molecule contains a group such as $H_{2}C$ = $CH_{2}$ or even cyclohexene would show an intense absorption system with a maximum intensity at about 180 nm. Acetylene group: the acetylene group shows an intense absorption system at about 190 nm and allylic group absorbs very strongly 225 nm.

A transition involving Ï€* --n promotion is useful in identifying a chromophore as it gives a characteristically weak absorption system which is usually of high wavelength of system due to Ï€* --n promotion and may be because it is interfered with by them.

Aldehyde group: the â€“CHO or aldehyde is useful chromophore showing Ï€* --n absorption system at about 280 nm quite like formaldehyde itself. Benzaldehyde group: the aldehyde part of a conjugated Ï€ electron system and cannot be treated as a chromophore.

The chromophore groups are.

 Chromophore Example solvent Ethylene Vapour Acetylene Hexane vapour Acetaldehyde Vapour and hexane Acetoxime Water Acetonitrile Vapour -COOH Acetic acid Water -NO2 Nitro methane Methanol -COOR Ethyl acetate Ethanol -CONH2 Acetamide Hexane and water -N=N- Azomethane Ethanol

## Example of Basic Chromophore

The basic chromophore include the azo compounds, the azine group and the indamine group. The azo group â€“N=N- is found in all azo dyes and in these compounds a benzene ring is attached to each nitrogen atom. The dyes in this group can be considered as the derivative as azobenzene.

The examples of biological stains having this chromophore are Bismarck brown, methyl red and methyl orange.

The azine group is mainly found in phenazines. The neutral red and the safranin are good example of azine stains.

The indamine group â€“N= is mainly found in indamines, the thiazines and other types of dyes. Many of the dyes have two benzene rings attached to a nitrogen atom.

The quinonoid structure is as follows.

The thiazines have the two benzene rings getting attached further together by sulphur atom apart from nitrogen atom. The most common and well known of these thiazine stains is methylene blue.

## Types of Chromophores

Overall there are four types of chromophores and they are all open tetra pyrroles better known as phycobilins. These are synthesised by heme synthetic pathways which are common to heme and Chl synthesis. A close tetra pyrrole ring is synthesised and then modified into side chains which yields four types of chromophores.
• Blue coloured phyco-cyano-bilin
• Red coloured phyco-erythro-bilin
• Yellow coloured phyco-uro-bilin
• The purple coloured phyco-violo-bilin
These compounds differ in the number of conjugated double bonds and these are directly related to their absorption capabilities. Apart from this the number of covalent bonds through which the chromophores are attached to apo-proteins also differ. Normally the apo-proteins attach at one site and linkage through two covalent bonds at specific binding sites.

Identical functional groups in different molecules do not absorb at exactly same wavelenght and the energy change for a particular transition determines the position of given group. The energy change depend upon the structural environment of the molecule and for qualitative interpretation of the spectra only the region above 200 nm are important.

The position and intensity of an absorption band of a simple chromophore can be modified by the attachment of certain groups in basic chromophore system known as auxochromes.

Independent chromophore:

When a single chromophore is sufficient to impart colour to the compound like azo group, nitroso group, quinonoid group, they are termed as independent chromophores.

Dependent chromophores:

When more than one chromophore is essential to bring about a specific colour in the chromogen then these are grouped under dependent chromophores.

Examples are >C =O group, >C = C< group etc. The acetone having one ketone group is colourless but diacetyl having two ketones is yellow while triketopentane with three ketonic groups is orange.

The series of diphenyl polyenes, having first three members are colourless while the tri form is yellow. The penta series has orange colour and with n = 11 the colour is found to be deep violet.

The intensity of colour increases because of the auxochromes and also make the chromogen a dye by fixing these to fibre by associating and by salt formation. The fixation happens due to the bond formation of chemical bond between fibre and auxochromes.

## Function of Chromophore

The chromophores are responsible for many functions apart from the molecular colour.